Water-cooled stator bars for electrical generators are comprised of a plurality of small rectangular solid and hollow copper strands brazed to one another to form a bar. The ends of the strands are brazed to an end fitting, typically referred to as stator bar clip. A cover is brazed to the clip window. The end fitting serves as both an electrical and a cooling flow connection for the stator bar.
The hollow end fitting typically includes an enclosed chamber for ingress or egress of stator bar cooling liquid, typically deionized water. At one end, the end fitting receives the ends of the strands of the stator bar. The fitting and the peripherally outermost copper strands of the stator bar are brazed to one another. The opposite end of the fitting is connected to a stator cooling conduit.
Liquid cooled stator bar clips have gone through design changes over the years. However, they typically contain mixed solid and hollow strands brazed to one another, and a cover brazed to a clip window. During operation, the hollow strands carry water to cool off the bar. Over time, leaks can develop about the connection between the stator bar ends and the stator bar fitting, between cover and clip as well as between adjacent strands. Leaks may also occur at various plumbing connections. It is believed that the major leak mechanism is a crevice corrosion process which initiates in the braze alloy at the interior surface of the braze joint. Crevice corrosion is a localized form of corrosion usually associated with a stagnant solution on the microenvironmental level. Such stagnant microenvironments tend to occur in crevices such as micro surface voids formed during brazing, especially at the boundary of strands and braze alloy. Crevice corrosion is initiated by changes in local chemistry within the crevice, such as shift to phosphorous acid conditions in the crevice. Stagnant water in the chamber of the fitting is in contact with the braze alloy and the copper strands. This coolant contact with the braze joint and cooper strands is believed to cause corrosion and consequent leakage.
Field repair of coolant leaks through the stator bar end connections has been successful. A leak site is identified by several different tests, such as vacuum decay and traceable Helium test.
An epoxy barrier coating method has been used as a leak repair and prevention method. An example of an epoxy barrier coating method is disclosed in U.S. Pat. No. 5,605,590, the disclosure of which is incorporated herein by this reference. This epoxy barrier coating has been applied to provide protection against water initiated corrosion mechanisms along the brazed length of the strand package. Epoxy coating is manually injected. The voids and air pockets are possible during injection. Thus, the process is labor intensive and requires 100% inspection. As a result, the process can be labor intensive, takes a long time to complete, and can produce defects.
There are also other leak issues in liquid cooled generators. Indeed, there are many types of leaks associated with water-cooled generators. Stator bar end crevice corrosion is the major one. But other leaks are caused by porosity, cracking and localized damage during manufacturing rather than as a result of corrosion.
There is a need for an improved method for leak repair and prevention in liquid-cooled generators. In particular, there is a need for corrosion protection at the junction between the stator bars and their clips. The corrosion protection should be robust and be applicable to various stator bar clip designs, including clips for recessed braze, flush braze and raised hollow strand braze designs. There is also a need for an improved method to repair and prevent leaks within the generator water pass that arise from other causes.